1. Field of the Invention
The present invention relates to an apparatus and method for transferring semiconductor wafers, between a cassette and a boat for heat treatment.
2. Description of the Related Art
Semiconductor devices, such as IC's or LSI's are manufactured by subjecting wafers cut from an ingot to various sequential steps, such as a surface treatment, thermal oxidation, impurity diffusion, film deposition and etching step, during which the semiconductor wafers repetitively undergo heat treatments (heating, CVD, oxidation, diffusion and other treatments) a plurality of times.
As a wafer heating furnace use is made of a vertically-stacked multi-stage furnace, such as a four-state furnace, to heat-treat the semiconductor wafers. A multi-stage stack unit is provided in front of the furnace and various automatic devices are mounted on the stack unit to treat the semiconductor wafers.
For the semiconductor wafers to enter or leave the heating furnace, a special quartz boat is employed in which case semiconductor wafers can be carried 200 pieces at max. per lot on a single boat. A special wafer-cassette (carrier) is used to convey semiconductor wafers to the heating furnace via a preliminary step. Usually, the cassette can hold 25 pieces of semiconductor wafers at max. per lot. Thus semiconductor wafers on a respective cassette are transferred onto one boat.
The wafer must carefully be handled so that no contamination occurs due to a direct touching of the operator's hand on the wafer. To this end, a special wafer transfer apparatus is located, as its auxiliary equipment near the inlet of the furnace to allow ready transfer to be automatically made from the cassette to the boat or from the boat to another cassette for washing.
Japanese patent application (Kokoku) 60-32352 discloses a conventional wafer transfer apparatus including a table carrying a boat and cassettes thereon and a loading device for handling the wafers between the cassette and the boat. The table extends in the X-axis direction, that is, in the direction of the axis of the furnace. Furthermore, the wafer transfer apparatus includes a travelling mechanism for travelling the table along the X-axis direction and stepping mechanism for stepping the table in the X-axis direction at a rate of one pitch corresponding to each groove which is formed on the boat to load the respective wafer.
The loading device includes a wafer-lifting mechanism liftably mounted below the table and is adapted to lift the wafers on the boat and cassette upwardly and a wafer-gripping mechanism having a pair of wafer chucks adapted to be opened and closed.
When the wafers in the cassette are to be transferred to the boat by the apparatus, a plurality of cassettes are arranged on a stage, all wafers in a first cassette being lifted up by the wafer-lifting mechanism away from the cassette, gripped by the chuck mechanism all at a time, carried on the boat and placed onto the boat with the chuck mechanism opened. The wafers on a second cassette et seq. are sequentially transferred to the corresponding boat in substantially the same fashion as set forth above. Then the boat carrying the wafers thereon is placed by an elevator device on a carrier located in front of the furnace, and loaded by a soft landing device into horizontal type furnace or by a boat loader into vertical type furnace.
The aforementioned sequential operation of the wafer transfer device is controlled, by a computer, based on a predetermined programming.
The cassette, boat and wafer chuck are of such a type that grooves are formed at the same interval to hold the respective wafer.
Since, however, wafers on the cassette are held all in the same direction on the aforementioned wafer transfer apparatus, the pattern formation surface and reverse surface of the wafers face each other if the wafers in the cassette are transferred to the boat all at a time. This sometimes offers a disadvantage in the case where the pattern formation surfaces of the wafers are arranged in an opposed relation called a "face-to-face" array or the reverse surfaces of the wafers are arranged in an opposed relation called a "back-to-back" array.
Furthermore, it is not possible to rapidly cope with the situation where there is a change in the pitch of those grooves of the boat where the wafers are held in place. If the groove pitch is changed, for example, from 3/16 inch to 3/32 inch, then it is necessary to change the groove pitch on the wafer chuck from 3/16 inch to 3/32 inch accordingly. For the chuck exchange, the task time is prolonged because the wafer transfer operation is interrupted over a long period of time.